This invention relates generally to piezoelectric transducers, and more specifically, to a method for control monitoring the polarization and volume of discrete layers in a piezoelectric transducer element.
Heretofore, nearly all of the transducer elements which must be artificially polarized to produce a piezoelectric effect have been uniformly polarized in the thickness or lateral dimension of the element. Conventional transducers of this artificially polarized category, i.e., ferroelectric ceramic materials such as barium titanate or lead zirconate--lead titanate or lead metaniobate, are polarized completely or uniformly through the element in a direction parallel with the applied polarizing potential or bias voltage.
Polarization of such ferroelectric ceramics refers to the physical condition of the crystals of the material such that its magnetic dipoles are aligned in a given direction. A material may thus be polarized in one of two different directions, i.e., the magnetic dipoles of the material may be aligned in one of two opposing directions, or the material may be unpolarized, a condition wherein the dipoles are arranged in random directions with respect to one another.
Theoretically, polarization of such a material is accomplished by applying a strong electric field to the material, and coincidentally heating it to a temperature above its Curie point. When the material is heated above its Curie temperature, it loses its ferroelectric properties, and the electric field aligns the magnetic dipoles of the material to the direction of the applied field. Practically, however, the material is heated to a temperature slightly below its Curie point, during which time a strong electric field is applied to the material. The material is thus allowed to polarize over an extended period of time. With the electric field still applied to the material, it is slowly cooled to an ambient temperature. When the external field is then removed, a remnant polarization is retained in the material and the ferroelectric ceramic now will typically respond in a manner similar to that of other natural piezoelectric materials such as Rochelle salt or ammonium phosphate crystals.
The effect which the polarized ceramic exhibits is known as the piezoelectric effect, which refers to the material's capability of mechanically deforming in response to an applied electrical signal, and conversely, or storing an electric charge in response to a mechanical or acoustical excitation. By placing electrodes on opposing surfaces of the element, this stored charge may be released in the form of an electrical current, the value of which is a function of the applied mechanical or acoustical force. The polarized ferroelectric element, which exhibits this piezoelectric characteristic, is thus a true transducer, a device capable of converting or transforming directly between various forms of energy, in this case between mechanical or acoustical energy, and electrical energy.
Conventional transducers made from such polarized ferroelectric materials, however, have several distinct limitations, one of the most important being their incapability of producing a broadband response to excitation. For instance, all piezoelectric transducers have natural resonant frequencies, wherein the transducer element vibrates in a ringing fashion after it is struck with wave energy of a certain frequency. A conventional fully polarized transducer has both a natural mechanical resonant frequency, and at least one natural electrical frequency of resonance. The period of this natural resonant frequency will equal to twice the velocity of the wave striking the element times the thickness of the element. If the element is excited by a one-quarter wave length or greater signal with respect to its period, the transducer will tend to ring, producing an undesirable output. This ringing response of a particular transducer will inherently limit the frequency range of adequate response to mechanical or electrical signals applied to it. Thus, the effective response frequency range of a conventional piezoelectric transducer is limited to those excitation frequencies which do not cause the transducer to respond in a ringing fashion. The conventional piezoelectric transducer is thus a narrow band transducer, and often rather limited in application.
Another significant limitation of current transducer technology is the inability to produce homogeneous transducer elements which may be partially polarized in their thickness direction, such that multiple layers of different phase, area, and depth of polarization may be achieved. For instance, it may be desirable to have bimorph or multimorph transducers which are comprised of multiple layers, each layer having a particular polarization in one direction or another, or alternatively being unpolarized. FIG. 8 shows a stacking of two such elements 14 and 15, the two elements being mounted on a stationary wall 19 and oppositely polarized. The application of a voltage to this configuration will cause the device to bend in one direction or the other, depending on the polarity of the voltage. This configuration is known in the art as a bender transducer. Other configurations or additional layers may be used to accomplish other results, such as high voltage generators.
Prior art transducer technology accomplishes this multimorph transducer structure by utilizing several distinct elements, polarizing them completely in one direction or another or maintaining them unpolarized, and then combining them in discrete layers and areas, as necessary to perform the desired function. This technique, of course, results in distinct interfaces between adjoining layers, and the inherent impedance mismatch at the layer interfaces often can create significant difficulties with respect to the desired response.
These disadvantages have been largely overcome by a method for controlling the phase, area and depth of polarization in a single piezoelectric element, a method which is more fully disclosed in copending application entitled "Method of Producing Transducers with Phase, Area, and Depth Controlled Polarization," by Norman F. Dixon and William D. Jolly and assigned to the assignee of the present invention.
To exercise control over specific transducer configurations, with respect to the phase, depth and area of selected element layers, as made possible by the technique of the copending application referred to above, the element itself must be monitored, preferably real time, during the fabrication of the multi-layered transducer. Such a method of monitoring, as disclosed and claimed in the present application, permits accurate control over the physical dimensions of individual polarized or unpolarized layers within a single transducer element, as well as an accurate indication of their state of polarization.
In view of the above, it is a general object of the present invention to provide a reliable method of monitoring polarization of piezoelectric transducers.
It is another object of the present invention to provide a method of polarization monitoring which is useful with respect to varying depths and areas of polarization in a single transducer element.
It is a further object of the present invention to provide a method of monitoring which can control the fabrication of multi-layered transducer elements in real time.